1. Field of the Invention
This invention relates to a 1-phase energized brushless motor which can start itself in a predetermined direction with a single position-detecting element. While a 1-phase energized brushless motor is sometimes called a 2-phase motor from its arrangement of armature coils, it is appropriate to call such a 1-phase brushless motor a 1-phase motor because it is basically energized by a 1-phase energizing method. Thus, the present invention relates to a brushless motor which is energized by a 1-phase energizing method.
2. Description of the Prior Art
A 1-phase or single phase energized brushless motor is used as an axial-flow fan motor and the like because it can be produced at a relatively low cost, since it requires only one position-detecting element.
In 1-phase brushless motors, normally a magnetoelectric transducer such as a Hall element or a Hall IC (integrated circuit) and a position-detecting coil are mainly employed as a position-detecting element. The former detects a magnetic N (north) or S (south) pole of a rotor magnet (commonly a main field magnet is used, and accordingly a magnet rotor is hereinafter referred to as a field magnet) to energize armature coils in a selected direction to rotate the brushless motor in a predetermined direction. To the contrary, in a 1-phase energized brushless motor employing a position-detecting element of the latter type, when armature coils are energized in a selected direction, a turning torque in either direction possibly may not be generated, and hence the motor may not be rotated, in a particular position of a field magnet relative to the armature coils.
It is well known in the art that a 1-phase energized brushless motor which employs a position-detecting element of either of the two types has a drawback that it cannot start itself.
For example, in a 1-phase energized brushless motor employing a magnetoelectric transducer, if the magnetoelectric transducer acting as a position-detecting element detects an intermediate portion of a field magnet between adjacent N and S poles upon starting of the motor, a signal to instruct in which direction electric current is to flow through the armature coils will not be produced from the element, and hence the armature coils cannot be energized properly and the motor cannot start itself.
On the other hand, in a 1-phase energized brushless motor in which a position-detecting coil is employed, the position of a field magnet cannot be detected upon starting of the motor, and hence it cannot be determined in which direction the motor is to rotate. Besides, it has a similar drawback to a motor employing a magnetoelectric transducer that the motor cannot start itself when armature coils are not positioned to generate a torque.
Thus, a 1-phase energized brushless motor cannot sometimes start itself, whichever type of position-detecting element is employed.
Accordingly, a 1-phase energized brushless motor generally has a magnetic member such as an iron bar disposed in a magnetic circuit to generate a cogging (reluctance) torque in order to stop a field magnet at a position from which the motor can start itself. Accordingly, a 1-phase energized brushless motor can be obtained which can start itself without fail if armature coils are energized upon starting.
Again, where a position detecting coil is used as a position-detecting element, if the motor is designed to utilize a cogging torque to allow self-starting thereof, it cannot be forecast in which direction the motor will rotate because the position of the field magnet cannot be identified as described above.
Further, if a 1-phase energized brushless motor whichever type of position-detecting element is employed, is designed to utilize a cogging torque to allow self-starting of the motor, where armature coils, the position detecting element and a cogging torque generating member are not positioned appropriately relative to one another, the motor cannot be of good performance.
In addition, in conventional 1-phase energized brushless motors, it is very troublesome to adjust relative positions of those members and much time is required for assembly and adjustment of those members, and hence the cost becomes high.